Low frequency welding control



Sept. 23, 1958 J. R. PARSONS LOW FREQUENCY WELDING CONTROL 4Sheets-Sheet 1 Original Filed Sept. 30, 1948 INVENTOR John R. Parsons.

ATTORNEY Sept. 23, 1958 J. R. PARSONS LOW FREQUENCY WELDING CONTROL 4Sheets-Sheet 2 Original Filed Sept. 30, 1948 w 5 I. 8 :B m .m R m Y SNMm. Ev om m M M n E R lU II I R O n 5 .3 n mm M n W .h .A m 7. .5 m mmflu Hm I o E m mm W mm 69 i -x .M W4 3 H W U vn zm m .5 3 I 2. R N m 3 3xx mm mm hm un WITNESSESZ 5 J M n Sept. 23, 1958 J. R. PARSONS LOWFREQUENCY WELDING CONTROL 4 Sheets-Sheet 3 Original Filed Sept. 50, 1948INVENTOR John R. Parsons.

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ATTORNEY 1 N: 2& 3. m l a u I k m9 335cm mum gm v 2: mm. 6. .5 mow a. 21 a w a: I m N& u ZN I lfmow PE w ll Now I 20w m fi m z $2 25m .w l EONSept. 23, 1958 J. R. PARSONS LOW FREQUENCY WELDING CONTROL OriginalFiled Sept. 30, 1948 4 Sheets-Sheet 4 2.22; oE t m 32: 8 29. /A7/A EVVWV/ A Q G5 2: y u s w co 09.35 g P V/ //////////m :v G: By W x? $5 215v9.3 \AWQ/ ////////////Z Q2. 5Q Q6 INVENTOR John R. Parsons MM ATTORN EYUnited States Patent fall Low FREQUENCY WELDING CONTROL John R. Parsons,Pennsauken, N. 1., assignor t0 Wesiing house Electric Corporation, EastPittsburgh, Pin, a corporation of Pennsylvania Original applicationSeptember 30, 1948, Serial No. 52,103, now Patent No. 2,619,591, datedNovember 25, 1952. Divided and this application April 26, 1952, SerialNo. 284,561

32 Claims. (Cl. 321

This invention relates generally to electronic discharge apparatus andhas particular relation to electronic timing systems for controllingsupply of power from a source to a load by means of electronic dischargeapparatus. in a specific embodiment the invention is applied toresistance Welders for electronically controlling the transfer ofcurrent from a power source to a welding load.

This application is a division of my application Serial No. 52,103,filed September 30, 1948, now Patent No. 2,619,591, dated November 25,1952, and assigned to Westinghouse Electric Corporation.

Resistance welders have, in the prior art, generally operated fromcommercial alternating current power lines, and have derived power froma single phase of such lines, despite the fact that power lines forindustrial electricity are generally of the three phase variety. Suchoperation has generally been considered satisfactory for relatively lowpower welding.

As the thickness of material to be welded increases the power requiredfor the welding operation likewise increases exponentially.Additionally, and especially in in the welding of steel, a reactivecomponent of current is introduced into the welding load by thereactance of the secondary winding of the welding transformer linkingwith the steel being welded. The reactance of the secondary winding ofthe welding transformer becomes higher generally than the resistance ofthat winding, including the weld, so that the power factor of thewelding load and consequently of the load drawn from the power line maybecome of the order of 25% or lower.

Still a further factor to be considered in welding extremely heavymaterials, requiring extremely high welding current, relates to skineffect in the welding circuit due to the flow of alternating current inthat circuit. One does not ordinarily consider skin effect to besignificant at 60 cycles per second, but under the conditions ofextremely high current'and extremely low direct current resistanceencountered in welding transformersecondaries, the resistance of thesecondary is determined in considerable part by skin eflect, and theresistance of the secondary circuit cannot, accordingly, be loweredindefinitely by ordinary engineering expedients, as, for example, byreducing the D. C. resistance of the circuit. Since the current requiredfor welding is fixed, the kilowatt demand of the welding machine isproportional to the resistance of the secondary circuit of the Weldingtransformer to alternating current, resulting in a high kilowatt demand.

Single phase welding has, accordingly, the following limitations:

(1) It causes unbalance of the three phase system from which power isdrawn.

(2) It creates a high kilovolt ampere demand at low power factor.

It is extremely desirable to decrease the kilovolt ampere load requiredby a given welding equipment since thereby-the cost of operation'of thewelder may be de creased, and further, since thereby the total cost ofan This represents an increased'cost to the user of the weldingequipment, and is obviously to be avoided.

When a current impulse of one polarity is applied to the primary windingof a single phase transformer, there is created in the iron core of thetransformer a magnetic field which is proportional to the current untilthe iron saturates. While the magnetic field induced by current flowingin the primary winding is changing, a voltage is induced in thesecondary winding of the transformer, and if the secondary winding isclosed or completed, current flows in that winding. When the secondarycurrent reaches its maximum value, the ratio of primary to secondarycurrent is determined by the turns ratio of the transformer to a firstapproximation, and when the primary impulse ceases the secondary currentdecay-s. A succeeding current impulse may then be applied to the primaryof the transformer in a direction opposite to the first impulse, so thattwo time adjacent impulses constitute in effect a low frequency cycle ofcurrent. By virture of the technique briefly described immediatelyabove, it is possible then to generate a low frequency alternatingcurrent by reversing at controlled times the direction of'flow ofcurrent. Reduction of frequency of such current, when used for Welding,reduces skin effect in the secondary of the welding transformer, sincethe latter depends directly upon fre quency of current. This, in turn,raises the effective conductivity of the welding transformer secondary,and thereby the kilowatt demand made by the welding equipment. windinglikewise is determined by frequency, being directly proportionalthereto, reduction of the frequency of the welding current reduces thereactance of the welding secondary and accordingly raises the powerfactor of the welding system.

It is further found, as an ancillary advantage, that the slow rise ofwelding current effected during low frequency welding causes the currentto distribute itself in the welding more evenly than is the case whenhigher frequencies are used, resulting in better welds and obviating thedifficulties caused by local heating orspitting.

The system of alternately reversing current welding above brieflydescribed lends itself to operation from a three phase power line,deriving current equally from all the phases of the line, since voltagewaves in the various lines of a three phase power system overlap inphase and, accordingly, super-position of half waves of current of onepolarity in a single secondary, provides impulses of current of onepolarity of an overlapping and substantially continuous character, eachphaseof the line providing current for of each cycle. Provision forreversal of this current deriving from athree phase line, and control ofthe amplitudes of those currents, may comprise arc discharge devices,and specifically ignitrons, connected in inverse parallel pairs in eachof the separate phases of the three phase lines, each ignitronbeingcontrolled by a firing tube, to which is applied firing signal, inaccordance with a predetermined law or timing schedule, the timingschedule being adjustable to provide a wide range of differentoperations.

Timing arrangements may be provided both for controlling the over-alloperation of the welding system, to

Furthermore, since reactance of the secondary enable sequencing of thecomplete welding operation, and also control of the on-ofi times of theignitrons during Welding, to provide a low frequency operation inselected and adjustable periods of current flow, that is, in alternatelyopposite directions for predetermined time intervals in each direction.

It follows that the development of three phase to single phase weldingequipments wherein the single phase Welding current is of low frequency,and is controllable in high degree in respect to frequency andmagnitude, will lead to increased utility of high power weldingequipment, leading particularly to economical operation and constructionof welding equipment operating at extremely high power.

It is, accordingly, an object of the present invention to provide anovel system of electric welding.

It is a further object of the invention to provide an improved system oflow frequency welding.

It is still a further object of the invention to provide an improvedsystem of welding which utilizes the three phases of a three phase powerline as a source of welding current.

It is another object of the present invention to provide an improvedsystem of welding which establishes subis tantially an equal load on allthe phase of a multi-phase It is, more specifically, an object of thepresent invention to provide a system of welding which operates from amulti-phase line, wherein sequential pulsations of current of oppositepolarity are employed for welding, the pulsations being derived equallyfrom the various phases of the multi-phase line and flowing successivelyin opposite direction for precisely equal time intervals.

It is, broadly stated, an object of the present invention to provide anovel system for controllably transferring povc ler from a multi-phasepower supply to a single phase It is a further broad object of thepresent invention to provide a novel system for controllablytransferring power from a multi-phase power supply to a single phaseload, by causing successive increments of current flow from successivephases of the line in a first direction for a controllable increment oftime, and for thereafter reversing the direction of current flow in theload circuit and controlling said current to flow for. a precisely likeincrement of time.

It is still a further object of the invention to provide a novel systemof supplying alternating current to a load from a multi-phasealternating current source and from the separate phases of that sourcein equal amounts, in pulses having a lower frequency than the frequencyof the source, the frequency of the current supply to the load beingadjustable over a wide range of values.

Briefly described, in accordance with the present invention power issupplied to a single phase welding load over a three phase to singlephase transformer, from a three phase line, a pair of back to backdischarge valves, specifically ignitrons, being connected between eachphase of the line and a primary winding of the three phase to singlephase transformerfor controlling and timing the transfer of power to theload.

The ignitrons are fired in conventional fashion by grid controlledfiring valves, specifically thyratrons, the firing valves being suppliedcontinuously with appropriate anode voltages from the three phase line,and being supplied with firing pulses from a welding timer, the presenceand the timing of the firing pulses determining the times of firing ofthe firing valves, and consequently of the discharge valves orignitrons, with respect to the phases of the voltages supplied to thedischarge valves by the line. The discharge valves and their associatedfiring valves may be grouped for purposes of ready reference, each groupcontaining one discharge valve of each of the back to back pairs and thevalves of each group operating to transfer current in the same directioninto the welding load. Accordingly, the discharge valves of one groupfire in succession at phase intervals until the operation of the groupis discontinued by failure of application of firing pulses thereto.Thereafter firing pulses may be applied to the remaining group ofdischarge valves, which fire in succession, one after the other, untilthe operation of the remaining group is in its turn discontinued. Thecurrent flow deriving from each group of valves considered as a unit,from the time of initiation of current flow by a first valve of thegroup to the time of termination of current fiow by the last valve ofthe group, constitutes, then, a half-cycle of low frequency weldingcurrent.

In systems of the above character, it is essential that the durations ofalternate half cycles of low frequency welding current be of preciselyequal duration. In practice, this requires that the same number ofignition tubes be caused to conduct for each half cycle of weldingcurrent. Any failure of equality of durations of half cycles of weldingcurrent results in an unbalanced current in the primary of the weldingtransformer, or otherwise considered, a D. C. component of current inthe primary of the welding transformer, which in the course of a shortinterval of time saturates the transformer, and thereby renders thewelding equipment ineificient or inoperative.

It is, of course, obvious that the ignitrons which provide current inthe welding load in one direction must be adjusted to provide current ofprecisely the same mag nitude as do the ignitrons which provide currentfor the opposite direction, since otherwise, despite the fact that thesame number of ignitron tubes conduct for each half cycle of weldingcurrent the total welding current applied to the welding load or flowingin the primary of the welding transformer during alternate half cyclesof welding current will not be equal and the transformer will saturate.

It is, accordingly, an object of the present invention to provide asystem for postively equalizing the flow of welding current in oppositedirections in a three phase to single phase welding system.

It is, more broadly described, an object of the invention to provide asystem for preventing transformer saturation in three phase to singlephase transfer of current from a three phase source to a single phaseload.

Current flow in the welding load may be considerated to consist ofalternate positive and negative pulses. In a three phase system, threeignitrons are allocated to production of positive pulses and threeignitrons to production of negative pulses, the ignitrons beingconnected in pairs across separate phases of the three phase line. inback to back relation, in a manner which is per se well known. Each ofthe ignitrons, moreover, is controlled in respect to its firing time bya thyratron tube which is connected with its anode to the anode of theasso ciated ignitron and with the cathode of the thyratron in serieswith the ignitor electrode of the associated ignitron. Firing of thethyratron is controlled by means of a firing control pulse generator,which may itself be a further thyratron. The system thus envisages theuse of control pulse thyratrons, one associated with each ignitron ofthe system, or, otherwise considered, the sys tern, when energized froma three phase power line. envisages three control pulse thyratrons forcontrolling the positive pulses, in the load, and three similarthyratrons for controlling the negative pulses in the load.

In accordance with the present invention, the control pulse thyratronsallocated to the positive pulses are caused to operate in sequence inunits of three, firing of the first of the three thyratronsnecessitating absolutely firing of the remaining two, in time sequence.The firing of the first of the thyratrons is accomplished in response toa control signal at a time following initiation of a welding cycle.Firing of the first control pulse thyratron results in application of acontrol pulse to the first firing thyratron, and accordingly results infiring of the first ignition. The control pulse which is communicated tothe first firing thyratron islikewise applied to the second controlpulsethyratron as a firing pulse therefor, and the latter, accordingly,fires when its plate voltage arrives at a proper value. The secondcontrol pulse thyratron, in firing, establishes a firing pulse for thesecond firing thyratron and also for the third control pulse thyratron.The latter in firing, establishes a firing pulse for the third firingignitron, the cycle of operations then ceasing unless a further firingpulse is applied to the first of the control pulse thyratrons. Thepositive pulse of welding current maybe made as long as desired by thesimple expedinet of continuing to supply the first of the control pulsethyratron with control pulses at the appropriate instants, and may beterminated at any time after the firing of the third control pulsethyratron by failing to supply the first control pulse thyratron with afiring pulse. The negative D. C. pulse is controlled in an identicalfashion by a further group of three control pulse thyratrons, operationof which is initiated in response to a control pulse which may occuronly after termination of a positive D. C. pulse in the load circuit,but which may be controlled to occur at various times thereafter, sothat a gap may be established, which may be of predetermined duration,between positive and negative ones of the D. C. pulses of the weldingfrequency.

The firing tube control circuits, which, in accordance with the presentinvention, determine the time durations and time separations of thepulses of alternate polarity which constitute the welding current,allows, then, the conduction of the main ignitrons in groups of threeonly, and neverless than three, and utilizes a single pulse to controlthe conduction-ofeachgroup of three ignitrons. The control pulsethyratrons may be energized over a phase shifting circuit, which may beutilized to determine the phase position with respect to the phase ofthe supply voltages at which the firing of the ignitrons may take place,and this position may be varied readily by adjustment of the phaseshifting device.

The function of timing the duration of the welding cycles and theinterval therebetween' is accomplished by means of a frequency controlcircuit which may be briefly described as follows:

The frequency control circuit in accordance with the present inventioninvolves four gaseous conduction devices, or thyratrons, all ofwhich-operate in parallel from a single phase of the three phase powerline which provides power for the welding circuit. Of the four tubes, afirst is maintained conductive normally, and is connected in series withtwo resistors, the latter being'in parallel with one another and eachbeing shunted by a timing condenser. While the thyratron is conductive,then, the series connected resistors each develop a voltage, thevoltages being separately utilized for'rendering the second and thirdelectronic discharge devices non-conductive. When the first electronicdevice ceases to be conductive, on the other hand, the charges developedin the condensers connected across the parallel resistors dischargethrough the resistors, respectively, maintaining continually decreasingvoltage drops across each of the resistors, which, when sufiicientlydecreased, serve to render the second and third electronic dischargedevices conductive.

The fourth electronic discharge device is normally maintainednon-conductive by'means of a self-biasing circult but is renderedconductive in response to initiation of weld time. In circuit with thefourth thyratron is connected a'first pulsing transformer whichgenerates the control pulses hereinbefore referred to, these'latterhaving the function of initiating flow of a half cycle of weldingcurrent between the phases of the three phase power line and the weldingload.

In series with the fourth thyratron is a parallel resistance andcondenser combination, the condenser acquiring full charge immediatelywhen the fourth thyratron ignites: The condenser is connected' with thefirst thyratron in such manner as to bias the latter off when 6 thecondenser is charged. Accordingly, the firing of the fourth thyratronterminates firing of the first thyratron and initiates the timingoperation of the timing circuits connected in series therewith.

After a predetermined time, which equals in the present embodiment of myinvention, two cycles of the supply frequency, the first of the twotiming circuits discharges sufficiently to enable firing of the secondthyratron. Firing of the latter terminates firing of the fourththyratron and thereby terminates generation of control pulses, andthereby fiow of current from the power line to the welding load in onedirection.

The second timing circuit is adjusted to require three cycles of thesupply frequency for decay of its charge to a value sufiiciently low toenable firing of the third thyratron. The latter contains in circuittherewith a pulsing transformer for generating control pulses forenabling transfer of current between the power line and the welding loadin a direction opposite to that controlled by the fourth thyratron. Thethird thyratron, accordingly, now commences to fire and is permitted tofire for two cycles of the supply frequency. At the end of that time,the condenser of the timing circuit connected in series with the fourththyratron, which is now non-conductive, discharges sufficiently so thatthe voltage thereacross is sulficiently low to again permit firing ofthe first thyratron. The firing of the first thyratron immediatelygenerates 01f biasing voltages in the two timing circuits in seriestherewith, and cuts on; the second and third thyratrons. Cutting off thethird thyratron terminates transfer of current between the power lineand the welding load in the second direction, while biasing oh thesecond thyratron removes hold-oif bias from the fourth tube and permitsthe latter again to conduct, initiating a further cycle ofcon trolpulses.

The fourth thyratron is provided with two control electrodes, one ofwhich serves as an off bias control in re sponse to current flow in thesecond thyratron, while the remaining control electrode is controlledfrom the welding sequence timer in response to initiation of weld time.Accordingly, the cycle of operations described immediately abovecontinues until termination of weld time, in response to which thesecond control electrode of the first thyratron is provided with offbias voltage and the cycle of operation terminates.

It will be realized that it is essential for proper operation of thepresent system that a welding current cycle be not interrupted prior toits normal completion time, in order to avoid saturation of the weldingtransformer. Weld time, in accordance with the present invention, iscontrolled by a weld time thyratron in series with a weld time delayrelay. Firing of the thyratron energizes the relay and effects closureof circuit contacts which are adapted and arranged to terminate the weldtime period. Timing of the welding period is accomplished independentlyof the timing of the separate half cycles of welding current, and by aseparate timing circuit, so that there is no assurance that a weldingperiod will end only at the termination, or after the termination of acycle of welding current, in the absence of special precautions to thatend.

In accordance with the present invention, therefore, the voltageestablished across the timing circuit which is in series with the fourththyratron, and which subsists for the duration of each welding cycle,and decay of which to a predetermined value signals termination of eachcycle of welding current, is applied to an auxiliary control electrodecontained in the weld time thyratron, and serves to prevent firing ofthe latter despite termination of normal weld time until aftercompletion of a cycle of welding current.

It is, accordingly, a further object of the present invention to providea novel timing system for preventing termination of a welding periodexcept after the termination 7 of a cycle of welding current, thereby toestablish flow of welding current in complete cycles only.

The novel features which I consider to be characteristic of my inventionare set forth with particularity in the appended claims. The inventionitself, however, both as to its organization and its method ofoperation, together with additional objects, advantages and novelfeatures thereof, will best be understood from the following descriptionof a specific embodiment thereof, especially when read in connectionwith the accompanying drawings, wherein:

Figures 1, 2 and 3, inclusive, taken together, provide a circuit diagramof an embodiment of the invention; and wherein,

Figure l of the drawings illustrate the arrangement of a plurality ofgaseous conduction devices with respect to the phases of a multi-phasepower line, for the purpose of controllably transferring current fromthe power line to a single phase load;

Figure 2 represents in schematic circuit diagram a con trol pulsegenerating circuit for controlling the successive transfer of currentpulses between the separate gaseous conduction devices of Figure l to asingle phase load, at controlled times and for controlled periods;

Figure 3 illustrates in schematic circuit diagram a frequency controlcircuit for determining the duration and the time separation betweenseparate direct current pulses, following each other alternately inopposite direction in the welding load and a welding sequence timerarranged in accordance with the invention, and;

Figure 4 is a timing sequence or flow sheet illustrating the time offiring of the various electronic control tubes illustrated in Figure 3,as well as the action of various timing circuits associated with theseelectronic control tubes.

, Referring now to Figure 1 of the drawings, the reference numerals L1,L2 and L3 identify the lines of a three phase power source, the separatephases of which supply power to ignitrons lTU and 2TU, 3TU and 4TU, STUand 6TU, arranged in back to back pairs, one pair across each phase ofthe source, and each pair supplying power to a separate one of primarywindings 11, 12, 13 of a transformer T, the single secondary winding 14of which supplies power to a welding load L.

Each of the ignitrons 1TU to 6TU, inclusive, is contolled by a thyratronfiring tube, the respective thyratron firing tubes, identified by thedesignations 1FT through 6FT, being connected each in series between theanode and the igniting rod of an associated ignitron, so that firing ofany thyratron initiates firing of the associated ignitron.

The control electrodes 15 of the firing tubes 'lFT through 6FT arenormally maintained biased back beyond the critical potential at whichfiring of the thyratrons may takeplace, by means of a D. C. voltageestablished at each of the firing tubes across a condenser 16. D. C.voltage is established across each of the condensers it? by rectifying avoltage derived across the lines L1, L3, and applied via lines 6 to theprimaries 17 of the transformers 18, having secondaries 19 connected inseries with the condensers 16 via rectifying unit 26. Firing pulses aresupplied to the firing tubes ItFT, 3FT and SFT via lines 21, 22 and 23respectively, and to firing tubes 2FT, 4FT and 6FT via lines 24, 25, 26respectively, the firing tubes, in the absence of firing pulses suppliedover the appropriate ones of lines 21 through 26, inclusive, being cutoff, thereby cutting off the associated ignitrons lTU through 6TU, andpreventing transfer of current to the welding load L.

It will be clear, then, that any one of thyratrons .llFT through 6FT,inclusive, and hence any one of ignitrons lTU through 6TU, inclusive,may be caused to fire by transferring an appropriate firing pulse ofsuitable polarity and magnitude, over the appropriate one of lines 21through 26, inclusive, while the anode of the ignitron is positivelypolarized. The firing of any one of ignitrons lTU, 3TU, STU willtransfer to the load L a current of one polarity, which may for purposesof convenience in explaining the present invention be denominatedpositive, and transfer of current via any one of ignitrons ZTU, 4TU, 6TUwill correspondingly cause flow of current in the load L in a directionopposite to the positive direction, and which may, therefore, bedenominated negative.

it will be realized that the utilization of ignitrons and thyrutro-ns inthe present invention involves a matter of choice of circuit elements,and that other types of electronic discharge devices may be employed inpracticing the invention without departing from the true spirit thereValves liFT through 6FT will be referred to hereinafter as firing valvefor the sake of generality, since these valves serve to initiate firingof the ignitrons. The ignitrons themselves may be referred to as such,or in the alternative as are discharge devices or valves, it being theirsole function to pass current to the welding load at times determined bythe firing tubes lFT through 6PT. and in particular, whenever the latterare firing. it will, accordingly, be evident that thyratrons or othergaseous conduction devices may be substituted for the ignitrons 1TUthrough 6TU, especially for operating into relatively light weldingloads, and that hard or purely electronic valves may be utilized inplace of the thyratrons EFT through 6FT if the required firing currentis of sufficiently low value.

Reference is now made to Figure 2 of the drawings wherein is illustratedschematically the circuit which develops control pulses for establishingfiring times for the firing tubes lFT through 6PT. Alternating currentanode potential for the control tubes 1CT through 6CT is supplied overthe three phase line L1, L2 and L3 via a transformer T2 having oneprimary winding 30 connected between lines L1 and L2, a second primarywinding 31 connected between lines L2 and L3 and a third primary winding32 connected between lines L1 and L3. The primaries of the transformerT2 accordingly are connected in delta with the three phase line L1, L2,L3. The secondary windings of the transformer T2, identifiedrespectively by the numerals 33, 34 and 35, which are associated in theorder named with the primaries 30, 31 and 32, respectively, are likewiseconnected in delta. Across the secondaries 33, 34 and 35 is a phaseshift device generally denominated by the reference numeral 36, andwhich consists of three mechanically ganged potentiometers 37, 38 and 39connected in delta with respect to the secondary windings 33, 34 and 35of the transformer T2. Connected across the potentiometer 36 are threeresistances 40, 41 and 42, which are connected in wye, therebyestablishing a neutral point 43 for the three phase system. The phase ofthe potentials established across the resistance 40, 41 and 42 withrespect to the neutral point 43 may be varied by varying the movablecontact 44 of the potentiometer 36, since the junction points betweenthe potentiometers 37, 38 and 39 are connected to midpoints 45 of thesecondary windings 33, 34 and 35 respectively of the transformer T2.

Three lines 46, 47 and 48 emanate from the variable taps 44 of thepotentiometers 37, 38 and 39. The voltages on the lines 46, 47 and 48are mutually displaced by a phase angle of since these voltagesoriginate in the three phase lines L1, L2 and L3, and the potentials onthe lines 46, 47 and 4S assume their positive maXima in succession, inthe order in which the lines have been named. The phases of the voltagesin the lines 46, 47 and 48 with respect to the voltages in the powerlines L1, L2 and L3 may be shifted by shifting the contacts 44, thesecontacts being ganged to assure that any variation of phase which isintroduced into one of the lines 46, 47 and 48 is likewise introducedinto the remaining ones of these lines.

The line 46 supplies anode potential to the control tube ICT via theprimary winding 50 of a control transformer 1T having two secondarywindings, '1 and 52. The line 47 likewise leads to the anode of thecontrol tube 3CT via the primary winding 53 of a control transformer. 3Thaving two secondaries 5.4 and 55. Line 48 supplies anode potential tothe control tube SCT via the primary winding 56 of a control transformer5T. While the tubes lCT, 3CT and SCT are supplied with anode potentialcontinuously, in phases which lag in succeeding ones of the tubes by120, the tubes are normally cut off, and prevented from firing, by meansof a bias potential applied to the control electrodes thereof from arectifier unit RX, to which is applied alternating current derivingacross lines L1,, L3 via a primary winding 60 of a transformer 61 havinga secondary winding 62. The secondary winding 62 of transformer 61 isconnected across two diagonally opposite terminals of .the rectifierunit RX, and D. C. output potential, is taken from the remaining twodiagonally opposite terminals of the rectifier RX, and applied across apair of series connected resistors 63, 64across which is. shunted asmoothing condenser 65. The mid-point of the resistors 63, 64isconnected' with the control electrodes of the control tubes 1CT, 3CT,5CT, in parallel, and the remaining terminal of the resistance 63 isconnected with the cathodes of the control tubes lCT, 3CT, SCT inparallel. Potential developed across the resistance 63 is utilized toestablish a negative potential on the control grids of the tubes lCT,3CT and SCT, which, in well known manner, prevents these tubes fromfiring regardless of the potentials which may be impressed on the.anodes of the tubes.

Firing potential is applied to the control tubes 1CT, 3CT and 5CT in amanner now to be described. Considering first the tube 1CT, firingpotential is applied to this tube over a pair of lines 66 in the form ofa pulse, derivation of which will be described hereinafter. Theoccurrence of a firing pulse on the line 66 initiates a cycle ofoperation of the control tubes ICT, 3CT and 5CT, and consequently, aswill be hereinafter described, of the ignitrons 1TU, 3TU and STU.Occurrence of a pulse on the line 66, accordingly, signals initiation ofa welding cycle. The pulse applied on the line 66 is applied to theprimary 67 of a transformer 1F, havinga secondary Winding 68 which isconnected with the control electrode of the control tube 1CT via arectifying unit 69, there being connected across the secondary winding68 and the rectifying unit 69, taken in series, a parallel combinationof resistance 71 and condenser 70. The rectifier unit 69 rectifies thepulse supplied via the transformer 1F charging the condenser 70 in suchsense as to render the control electrode of the control tube 1CTpositive to an extent sufiicient to establish ionization of the gas inthe tube ICT and consequently firing of the latter when the anode of thecontrol tube 1CT goes positive. The time constant of the combination ofcondenser 70 and resistance 71 is sufiiciently short to allow rapiddecay of the charge of the condenser 70 after initiation of firing ofthe tube 1CT. The timing of the firing pulse established on the controlelectrode of the tube ICT is synchronized in respect to the anodepotential applied to the same tube, since the control pulse applied tothe tube ICT is derived from the same phase of the lines L1, L2, L3 "asis the anode potential for the tube 1CT. The relative phase of thefiring pulse applied to the control electrode of the control tube 1CTwith respect to the anode potential applied to the tube ICT is, ofcourse, variable since the firing pu'lse occurs at a fixed time withrespect to the voltage established on the supply lines L1, L2 and L3,while the anode potential applied to the control tube ICT isapplied tothe latter via a phase shifting network 36, the latter being advanced bya variable time with respect to the'tirning of the firing pulse. Firingofthe tube ICT establishes a pulse of current in theprimary winding50'of the transformer- 1T, this pulse being transferred, first, via thesecondary 51 of the transformer IT to the grid pulsing circuit 72 of thecontrol tube 3CT, which is in all respect identical with the gridpulsing circuit illustrated in conjunction with the tube. lCT, theoperation of which has been described hereinbefore. The pulse appliedvia the secondary winding 51 accordingly is rectified and applied to thecontrol electrode of the control tube 3CT, establishing a firing timefor thatitube. Anode potential for the tube 3CT is. supplied theretoover the line 47 and the primary winding 53 of the transformer 31.

Accordingly, the controltube 3CT will conduct current at a timefollowing the firing of the tube ICT, this time being established by thetime of application of positive potential to the anode of the controltube 3CT. Firing of the tube 3CT establishes a pulse of current in theprimary winding 53 of the transformer 3T, which is transferred via thesecondary winding 54 of the transformer 3T to pulsing circuit.73 ofthe.control electrode of the control tube SGT, the latter pulsingcircuit again being identical with the pulsing circuit 72 andestablishing a pulse voltage for application to the control electrode ofthe control tubeSCT.

Application of positive control pulse to the control electrode of thecontrol tube SCT'establishes current flow in the latter when the anodepotential of the latter becomes positive, anode. potential being appliedto the control tube SCT via the line 48 and the primary winding 56 ofthe transformer 51. The pulse of current established in the primarywinding 56 is not re-transferred back to the first one of the controltubes, lCT, but the sequential operation of the control tubes ICT, 3CTand 5CT now terminates, unless a further control pulse is applied to theprimary winding 67, and the transformer 1F, via the line 66. Should sucha further control pulse be established a further firing sequence willoccur, the tubes iCT, 301 and 'SCT firing in sequence in a further cycleof operation, duplicating the sequence of operations above described.

Associated with the primary winding 50 of the transformer 1'1 is asecondary winding 52 which is connected via the lead 21 with the controlelectrode and cathode of firing tube lFT which determines the time offiring of the ignitron 1TU, establishing a firing potential on thecontrol electrode of the tube 1PT. at a proper time to. enable firing ofthetube, that is, while the anode of the tube 1FT is positive. Firing ofthe tube EFT establishes a pulse of current through the ignitorelectrode of the ignitron ITU, which establishes firing of the ignitron1TU, enabling transfer of apulse of current to the welding load L .viathe primary winding '11 and the secondary winding 14 of the transformerT.

When the control tube 3CT fires, likewise, the pulse of current in theprimary winding 53 of the transformer 3T transfers voltage tothesecondary winding 55 of the transformer 3T, the pulse of voltage beingapplied via the line 22 to the control electrode of the firing tube 3FT,which in turn establishes a firing time for the ignitron STU, andconsequently a positive pulse of current in the welding load L, via theprimary winding 12 of the transformer T, and the secondary winding 14 ofthe latter.

Firing of the control tube SCT likewise establishes a pulse in theprimary winding 56 of the transformer 51, which is translated into avoltage pulse in the secondary winding 57 of the latter, this voltagepulse being transferred via the line 23 to the control electrode of thefiring tube SFT, which thereupon fires and establishes firing of theignitron STU. The latter, in turn, transfers a positive pulse to thewelding load L via the primary winding 13 of the transformer T, and thesecondary winding 14 thereof.

In summary then, each transfer of a pulse to the line 66 establishes asequential operation of the control tube lCT, 3CT and SCT, the latterproviding control pulses in sequence to the firing tubes lFT, SF! andSFT, which 11 cause firing of the ignitron 1TU, 3TU and STU, insequence, and with phase separation of 120, thereby to establish apositive pulse in the welding load for a time equal to 360 of the supplyfrequency. The system then requires and enforces firing of the ignitronslTU, 3TU and STU in groups of three, in response to the application of asingle control pulse on the line 66, and the firing sequence of theignitrons lTU, 3TU and STU cannot be interrupted or disestablished, onceit has been initiated, until all three ignitrons have been fired. One ormore sequences of operation may be initiated in like manner byapplication of succeeding control pulses to the line 66, andinterruption of welding current may be accomplished at any time aftercompletion of a complete firing sequence of the ignitrons 1TU, 3TU. STU,by failure to supply a control pulse to the line 66.

The control tubes 2CT, 4CT and 6CT operate in a manner entirely similarto that described above as applying to the control tubes 1CT, 3CT andSCT, initiation of firing of the tubes 2CT taking place in response toapplication of a control pulse to the lines 30, which establishes afiring control voltage in the control electrode circuit of the controltube 2CT, to which is normally applied a negative off-biasing potentialestablished across the resistance 82 by rectification in the rectifierunit 33 of alternating current supplied over the line 84 from thesecondary winding 85 of a transformer 86 having a primary winding 87,which is, in turn, connected across the secondary winding 62 of thetransformer 61. The transformer 86 includes two additional secondarywindings 88 and 89, which are utilized to establish, in a like manner,off biasing potentials for the control electrodes of the control tubes4CT and 6CT, the control tube 4CT including in its grid circuit aresistor 91 connected in series with rectifier unit 91 and with thesecondary winding $8 of the transformer 86, and the control electrode ofthe control tube GCT containing in circuit a bias resistance 92connected in series with a rectifier unit 93 and with the secondarywinding 89 of the transformer 86.

Connected in series with the cathode circuit of the control tube ZCT isa primary winding 94 of a transformer ET, the latter having a secondarywinding 95 which is connected with the control electrode circuit of thecontrol tube 4CT by means of a pulse rectifying circuit 96, which isidentical with the pulse rectifying circuit associated with the controltube ICT, and with the pulse rectifying circuit associated with thecontrol circuit associated with the tube 2CT, and which has beendescribed in detail hereinbefore. Accordingly, firing of the tube 2CT isfollowed by firing of the tube 4CT, upon establishment thereat of asuitable positive anode potential. There is likewise connected in thecathode circuit of the tube 4CT primary winding 97 of a transformer 1T,a secondary winding 98 of which is applied to the input circuit 99 ofthe control tube 6CT, the latter being identical with the controlcircuits 96 and 81 associated with the control tubes 4CT and ZCT,respectively. Accordingly, firing of the tube 4CT is accompanied byapplication of a firing pulse to tube 6CT, which fires when the anodepotential thereof attains a suitable positive value.

Firing of the control tube 6CT terminates the firing cycle of the tubes2CT, 4CT and 6CT unless a further control pulse is applied over the line81 to the input transformer 2F of the control tube ZCT.

included in the transformer 2T is a secondary winding 1th) which, inresponse to a current pulse in the primary winding 94 of the transformer2T, translates a trode of the firing tube 4FT, which fires in responseto the voltage pulse, and in firing causes firing of the ignitron 4TU.Similarly, a secondary winding is magnetically coupled with the primarywinding of the transformer 6T, which is connected in series with thecathode circuit of the control tube 6CT, so that firing of the controltube 6CT accomplishes transfer of a voltage pulse via the line 26 to thefiring tube 6FT, which breaks down in response to a control pulseestablished on the line 3t}. the control tubes ZCT, 4CT and 6CT breakdown in sequence, transferring firing pulses to the firing tubes ZFT,4FT and 6FT.in sequence, and these latter, in firing, accomplish firingof the associated ignitrons ZTU, 4TU and 6TU in similar sequence,applying overlapping negative pulses of potential to the negative load Lvia the primary windings 11, 12 and 13 of the welding transformer T. Theignitrons 2TU,- 4TU and 6TU accord ingly fire in sequence, in groups ofthree, in response to a single control pulse applied to the leads 8 1,and successive firing sequences take place only in response tosuccessive applications of pulses to the line t firing terminating afterfiring of the tube 6TU, unless a further firing sequence is initiated.

We now consider the manner in which control pulses are established forapplication to the lines 66 and 80. Referring specifically to Figure 3of the drawings, there is disclosed a circuit diagram of an apparatusada ated for providing control pulses to the firing pulse generatorcircuit of Figure 2 of the drawings. Power for the control pulsegenerating circuit is supplied from three lines 110, 111 and 112, whichare connected respectively with opposite ends and with a center tap of asecondary winding 103 of a transformer 194, the primary of which isconnected between lines L1 and L2 (see Figure 3).

Upon initiation of a Welding sequence and in accordance with the wellknown practice in the art, the welding electrodes are clamped to thework for a predetermined time known as squeeze timc before current istransferred through the weld by the welding transformer. in the presentembodiment of my invention, during squeeze time, the line 111a is closedvia the normally closed contact 113. Accordingly the auxiliary controlelectrode 114 of the thyratron 115 is supplied with operating voltage ofalternating current in character. driving the potential of the controlelectrode 114 alternatively above and below the potential of the cathode116 of the thyratron 115, the latter being established by the lead 112.Capacitor 117, connected in circuit with the control electrode 114,accordingly charges by grid rectification in such polarity as tomaintain the control electrode 114 negative with respect to the cathode116 of the thyratron 115, serving thereby to maintain the thyratron 115in unfired condition. With the termination of squeeze time, the contacts113 are opened removing'potential from the line 111a. There is then novoltage available for charging the condenser 117 by grid rectification,and the condenser 117 discharges through the resistor 118 shuntedthereacross, placing the thyra tron 115 in firing condition uponapplication to the anode thereof a suitable value of positive voltage.

Reference is made to Figure 4 of the drawings wherein there is provideda timing diagram by reference to which the operation of the system ofFigure 3 may be clarified. In Figure 4 operations take place in a timesequence proceeding in alphabetical order, cross hatched portion of thediagram representing tube conduction, or condenser charge and decay, asthe case may be. The alphabetical designations which appear in thefollowing description refer to various portions of the curves shown inFig. 4.

So long as the thyratron 115 is in unfired condition the condenser 120contains no charge, and consequently no voltage exists thereacross. Thecondenser 12% can be charged only via a circuit including the secondarywinding 121 of a transformer 122, having a primary winding 123 connectedacross lines L1 and L2, this secondary winding being connectedfurthermore, to one anode 124 of double diode 125 having a cathode 126,in the cathode circuit of which is connected the thyratron 115.Connected in series across the secondary winding 121 of the transformer122 is a resistance 127 and a condenser 128, which together provide aphase shifting circuit for the voltage established across the secondaryWinding 121, phase shifted voltage being available at he terminal pointof connection between the resistance 12'? and the condenser 128, andthis voltage being applied to the control electrode 129 of a thyratron130. Cathode 131 of the thyratron 130 is connected directly with theline 112. The anode 132 of the thyratron 130, onthe other hand, isconnected in the cathode circuit of a double diode 133, having one anode134 which is connected with the line 110 viaa parallel combination ofresistance 135 and condenser 136.

it will be recalled that the lines 110, 111 and 112 are energized fromacross the lines L1 and L2, as is also the transformer 122. Accordingly,thephase of the voltage applied to the anode 134 of the diode 133, aswell as to the anode 132 of the thyratron 130, is in phase with thevoltageapplied to the primary winding 123 of the transformer 122. Firingvoltage applied to the control electrode 129 of the thyratron 130,however, is derived from the phase shifting circuit comprisingresistance 127 and condenser 128, and firing voltage accordingly lagsbehind anode voltage at the thyratron 13% causing firing of the latter(A) late in the anode potential cycle. The double diode 133 comprises afurther anode 137, which is connected with the line 110 over aparallelconnected combination of resistance 138 and condenser 139. Sincethe thyratron 130 fires once in each cycle of the potential appliedacross the lines 1111 and 112, the condensers 136 and 139, connectedrespectively in the anode circuits 134 and 137 of'the double diode 133,charge, (B) establishing negative potentials at the anodes 134 and 137,which are applied respectively to the control electrode 140 of thethyratron 141, and to the control electrode 142 of the thyratron 143,maintaining the thyratrons 141 and 143 in cut-off condition.

It will be clear then, that while the contacts 113 are closed thyratron1311 is firing, but that thyratrons 141, 115 and 143 are maintained incut-off condition. (See Figure 4.) Upon opening of the contacts 113,which signals initiation of weld time as has been mentionedhereinbefore, the charging voltage for the condenser 117 is removed, andthe latter rapidly discharges through the associated parallel connectedresistor 118, attaining a potential such that the control electrode 114of the thyratron 115 may fire upon application thereto of suitable anodepotential, assuming establishment, initially, of a suitable firingvoltage at the control electrode 144 of the thyratron 115.

Firing potential for the control electrode 144 is obtained over a phaseshifting circuit which is energized from the secondary of the filamenttransformer 145, which otherwise provides heating current for thecathode 116 of the thyratron 115. The secondary winding 145 of thefilament transformer of the thyratron 115 is connected via a lead 146and a resistor 147 to one terminal of a condenser 148, the otherterminal of which is connected with the line 112, and the firstmentioned terminal of the condenser 148 is further connected via a lead149 and over a resistance 149a with the control electrode 144. Theremaining terminal of the secondary winding 145 of the filamenttransformer for the thyratron 115 is tied directly to the line 112.Accordingly, energization of the filament 146 effects establishmentacross the condenser 148 of an alternating voltage phase shifted aheadof the anode voltage applied to the anode of the thyratron 115, phaseshifting occurring by virtue of the traverse of a current proportionalto filament voltage through the resistance 147, and the condenser 148 inseries, the potential across the condenser 148 leading the currentpassing therethrough in accordance with principles which are per so wellknown.

Firing of the thyratron (C) enables transfer of current in the doublediode 125, potential for the anode 124 of which is provided by thesecondary winding 121 of the transformer 122, and hence transfer ofcontrol pulses over leads 80, via anode 124a of double diode 125.Passage of current via the section of the double diode 125 comprisingthe anode 124 results in charging (D) of the condenser 120, the negativeterminal of which is connected via the secondary winding 121a,resistance 127 and the lead 150, with the control electrode 129 of thethyratron 130, biasing the latter off and initiating thereby discharge(E) of the condenser 136 over the resistance 135, and of the condenser-139 (E) over the resistor 13%. The relative rates of decay in thecondensers 136 and 139 are so established'that the condenser 136discharges the more rapidly, so that firing poten tial is introduced tothe; control electrode 141) of the thyratron 141 (F) by one full cycleahead of the time that firing potential is introduced from the anode 137of the doubel diode 133 to the control electrode 142 of the thyratron143. Accordingly, the thyratron 141 conducts before the thyratron 143conducts. Firing of the thyratron 141 results in charging of thecondenser 14%, the negative terminal of the condenser 148 beingconnected with the control electrode 144 of the thyratron 115.Accordingly, the thyratron 115 is cut off (G) at a time determined bythe discharge time of the condenser 136. in the present embodiment of myinvention, dis charge time of the condenser 136 is so established thatafter transfer of two cyclesof alternating current through the thyratron115, the potential of the control electrode of the thyratron 141 israised sufficiently to enable firing (H) of that tube, which establishescut-off bias thyratron 1 15 across condenser 148 without delay, so thatthe thyratron 115 is provided with time for passing a total of twocycles of alternating current.

It will be recalled that the condenser 139 discharges (E) more slowlythan does the condenser 136, and specifically that the condenser 139discharges to an extent suificient to enable firing of the thyratron 143at a period one cycle after firing of the thyratron 141. Accordingly,after the thyratron141 is fired and the thyratron 115 has been cut off,one further-cycle of alternating voltage occurs and then the condenser139 having sufficiently discharged, the thyratron 143 fires (I)establishing pulses of current on the line 66.

While the thyratron 143 is passing pulses of current, the thyratron 115remains cut otf. Since the thyratron 115 is now not conducting current,the condenser 121) is not being charged and the charge existing on thecondenser slowly leaks off (I) via the associated resistor 129a which isconnected in shunt with the condenser 12-9. Discharge time for thecondenser 120 is established at three cycles of the supplied frequency.After the condenser 120 has sufliciently discharged, i. e. in a timeequal to three cycles of the supply frequency, the thyratron 131 is nolonger biased off, and again fires, rapidly charging the condensers136(L) and 139(L) and cutting off thyratrons 141 and 143.

The circuit illustrated in Figure 3 of the drawings is now in itsoriginal condition, and if the welding cycle has not been terminated thecondenser 117 remains uncharged, since the co-ntacts 113 remain open.The thyratron 141 being new blocked, the condenser 148 which provideshold-off bias for the thyratron 115 discharges (M). Upon discharge ofcondenser 148, which requires a time equal to one cycle of the supplyfrequency, the thyratron 115 again discharges (N), and the entire cycleof operation repeats. The cycle of operation will repeat indefinitelyuntil such time as the contacts 113 are again closed, applying potentialto the control electrode 114 over the charging condenser 117. Withcontrol potential ap- 15 plied to the control electrode 114 over thecondenser 117 and after the thyratron 115 has ceased firing, a holdoffbias will be developed by the condenser 117 for the control electrode114 by grid rectification, and the cycle of operations will then ceaseuntil the contacts 113 are again open.

There is now presented the problem of preventing closure of the contacts113 until a complete cycle of operation has been completed by thecontrol pulse generator illustrated in Figure 3, that is, until asequence of two control pulses have been transferred over the line 30followed by a further sequence of two control pulses over line 66, aswell as by firing of three ignitrons in response to each of the pulses.

In order to assure that a welding cycle will not be terminated beforecompletion of a complete cycle of low frequency welding current, thefollowing is provided. It will be recalled the condenser 120 is chargedwhen the thyratron 115 fires, and retains its charge until comple tionof a cycle of operations, thereafter discharging, and that it is thedischarge of the condenser 124) to a suitable value which enables firingof the thyratron 134D and consequent recharging of the condensers 136and 139 signaling termination of a cycle of welding current.Accordingly, the condenser 120 may be assumed to be charged for theduration of a cycle of welding current and to be discharged at thetermination of the cycle. A lead 160 is provided, accordingly, whichcommunicates conductively with the negative terminal of the condenser12% over an obvious circuit, and which applies the negative potential ofthe condenser 12%, while the latter is charged, to an auxiliary controlgrid 161 of a thyratron WT which, when energized, signals the end of awelding period. Since the negative potential provided by the line 161maintains the auxiliary control electrode 161 of thyratron WT at anegative potential over a full cycle of welding current, the thyratronWT cannot fire until the end of a cycle of welding current, but is freeto fire thereafter upon application to the control electrode 162 thereofof suitable firing potential. In the normal operation of the thyratronWT firing potential may be applied to the control electrode 162 atrelatively random times, that is, at times which are not positivelyrelated to the condition or phase of the welding current. In accordancewith my invention then, application of firing potential to the grid 162is powerless to cause cessation of a welding period if the controlelectrode 161 is being then supplied with negative bias, signaling thata cycle of welding current had not terminated. After application to thecontrol electrode 162 of firing potential, the thyratron WT is preventedfrom firing by negative potential on the control electrode 161 untilcompletion of a cycle of welding current, at which time the potential ofthe control electrode 161 has risen positively sufficiently to enablethe thyratron WT to conduct.

it is further essential for the proper functioning of. welding equipmentembodying my inventions that termination of a welding period likewiseterminate operation of the pulsing generator which controls firing ofthe ignitrons llTU through 6TU. For this purpose a pair of contacts 1 53is introduced intermediate the cathode 126 and the double diode 125 andthe anode of the thyratron 115, the normally closed contacts 163 beingopened in response to energization of the relay WTD (Fig. 3) whichcontrols weld time and which is energized in response to firing of theweld time thyratron WT. Accordingly, after the thyratron WT has fired,signaling termination of the weld period, and the relay WTD has beenenergized in response to such firing, the contacts 163 open, opening theanode circuit of the thyratron 115 and disabling completely the controlcircuit which generates control pulses for initiating operation of theignitron 1TU through 6TU.

Refere ce is now made particularly to Figure 3 of the drawings whereinis illustrated schematically the circuits required for controlling thesequencing of a Welding system arranged in accordance with the presentinvention. Power for the sequencing equipment is derived across lines L1and L2 of the three phase power line to which is connected the primaryof a transformer 1614, the secondary 103 of which, as has been explainedhereinbefore, is utilized for establishing alternating potential on thelines 110, 111, 112. Connected between the line 179, which ties directlyto the line 110, and the line 112, which is connected to the center tapof the secondary winding 18% of the transformer 104, is a control relayCR in series with a manually operable switch SW. Closure of the switchSW, accordingly, energizes the control relay CR, which pulls up, closingthe normally open contacts 1'71 and 172 and opening the normally closedcontacts 173. Closure of the contacts 171 completes a circuit from theline via the squeeze time delay relay STD and the squeeze time thyratronST to the line Accordingly, anode potential is applied to the anode ofthe thyratron ST, and squeeze time delay relay STD is arranged to beenergized by firing of the squeeze time thyratron ST. Closure of thenormally open contacts 172 establishes potential on a line 174, thispotential performing a function which will be explained hereinafter.

Prior to energization of the control relay CR, and consequently prior toopening of the contacts 173, hold-off potential applied to the controlelectrode of the squeeze time thyratron ST is derived via the followingcircuit. The pair of resistances 175 and 176 are connected across thelines 177 and 112 in series. The line 177 is connected over a droppingresistance 178 with the line 116, and corn sequently with one terminalof the secondary winding 103 of the transformer 104. To the terminal 179joining the resistances 175 and 176, and extending between that terminaland the line 111 over normally closed contacts 173 of relay CR is afurther pair of series connected resistances 180 and 131. Since thepotentials on the lines 177 and 111 are oppositely phased with respectto the potential of the line 112, the latter being connected to thecenter tap of the secondary winding 1631, and the lines 177 and 111 toopposite ends of the secondary winding 1%, the potential applied to thecontrol electrode of the squeeze time thyratron ST and deriving from theterminal 182, forming the junction point between the resistances 180 and181, is of such phase as to maintain thyratron ST normallynon-conductive, and the potential established between the terminal 182and the line 112 furthermore serves to charge the condenser 183 by gridcircuit conduction in the thyratron ST, in such fashion as to establisha steady negative potential for the thyratron ST, serving to maintainthe latter in cut-01f condition even in the absence of the alternatinghold-off bias applied thereto.

Upon opening of the contacts 173, however, in response to energizationof the control relay CR, application of alternating voltage to thecontrol electrode of the squeeze time thyratron ST from the terminal 182ceases. The potential applied to the control electrode of thyratron STnow derives from the terminal point 179, and is in phase with thepotential applied to the anode of the thyratron ST. The alternatingpotential now applied to the control electrode of the thyratron ST isnot, however, of sufficient magnitude to enable firing of the thyratronST, since the alternating voltage, even at its positive peaks, is not asgreat as the steady bias potential established by the charge on thecondenser 183.

The charge on the condenser 183, however, now proceeds to leak off overa leak or discharge path comprising a fixed resistance 184 and avariable resistance 185, the resistance setting of the latter serving todetermine the discharge time of the condenser. Accordingly, after a timedetermined by the setting of the variable resistance 185 the charge onthe condenser 183 leaks off to a sufficient extent to enable asucceeding cycle of alternating potential at the contact 171 to causefiring of the thyratron ST, whereupon the latter fires, energizing thesqueeze time delay relay STD and signaling end of squeeze time in thewelding sequence.

Anode potential for the weld time thyratron WT and for the weld timerelay WTD is derived from the line 110 via the contacts 171, now closed,since the relay CR is energized, over the rectifier unit 186, throughthe winding of the weld time delay relay WTD, and over the normally opencontacts 187, and directly to the anode of the weld time thyratron WT.Upon energization of the squeeze time delay relay STD, however, thecontacts 187 are closed and anode potential is then applied to the anodeof the weld time thyratron WT via the weld time delay relay WTD. TheWeld time thyratron WT is associated with a control circuit, connectedwith the control electrode 162 thereof, and comprising a pair of seriesconnected resistors 188 and 189 which are connected between the lines177 and 112, the terminal point 190 between the two resistors beingconnected further in series with a pair of resistors 191 and 192, andthence via normally closed contacts 193 with the line 111. Controlelectrode 162 is connected in series with a charging condenser 194 andwith the terminal point between the resistors 191 and 192. Accordinglywhile the contacts 193 are closed, as they normally are, the potentialapplied to the control electrode 162 of the weld time thyratron WT is inopposite phase to the voltage applied to the anode of that thyratron,maintaining the thyratron cut olf, and further serving to charge thecondenser 194, by grid conduction. The condenser 194 is connected inseries between the terminal point between resistors 191 and 192 and thecontrol electrode 162, the charge thereon being in such sense as toestablish a steady negative bias on the control electrode 162.

Opening of the contacts 193 in response to energization of the squeezetime delay relay STD disconnects the resistance 192 from the line 111,and serves to establish an alternating voltage on the control electrode162 of the thyratron WTD, which is derived from the junction point 190,and which consequently is in phase with the anode potential applied tothe anode of the weld time thyratron WT. Nevertheless, the thyratron WTis not permitted to fire since the alternating potential applied to thecontrol electrode 162 from the terminal between resistors, 191 and 192is of insufficient magnitude to overcome the steady negative biasprovided by the condenser 194. Disconnection of resistor 192 from theline 111, however, has further prevented additional charging of thecondenser 194 and the latter now proceeds to discharge over a pair ofseries connected resistors 195 and 196, the latter resistor beingadjustable in magnitude to enable predetermination of weld time bydetermining the total time required for discharge of the condenser 194to a value suiiiciently low to enable the positive peaks of potential atthe junction point, 190 to cause firing of the weld time thyratron WT.

Energization of the squeeze time delay relay STD, which signals the endof the squeeze time period, and commencement of the Weld time period,further opens the contacts 113 in the line 111 thus removing chargingpotential from the condenser 117 and initiating generation of weldcontrol pulses by the circuit of Figure 3, in a. manner which has beenfully explained hereinabove.

After discharge of the condenser 194 to a sufficient extent, thepotential on the control electrode 162 of the Weld time thyratron WTdecreases to an extent suflicient to enable firing of the thyratron inresponse to the potential of the terminal point 190, provided, however,that the auxiliary control electrode 161 of the thyratron WT is notbiased negatively to an extent suflicient to prevent such firing. As hasbeen explained hereinabove, the auxiliary control electrode 161maintains its negative potential for the duration of a welding cycle andat the termination of the Welding cycle goes sufiiciently positive toenable the thyratron WT to fire, if the control electrode 162 is at thattime provided with a firing potential. We may 18 assume, then, thatafter a sufficient time has elapsed the electrode 162 has attained abias such as to enable firing of the thyratron WT, and that the end of awelding cycle of low frequency alternating current has occurred, wherebythe auxiliary control electrode 161 rises to firing potential, and thatthe thyratron WT then fires, terminating the welding period.

Firing of the weld time thyratron WT energizes the weld time delay relayWTD which now pulls up opening the normally closed contacts 163, which,as has been explained hereinabove, are connected in series between thecathode 126 of the double diode 1 25 and the thyratron 115, completelydisabling the circuit, illustrated in Figure 3, which provides thecontrol. pulses for the firing tubes and for the ignitrons, and therebypreventing any further transfer of current from the three phase linesL1, L2, L3 to the welding load L, via the welding trans; former T. a v

Additionally, energization of the weld time delay relay WTD effectsopening'of the normally closed contacts 197 which serves to initiatehold time. Hold time thyratron HT is normally biased off in a mannersimilar to that described in connection withthe operation of the squeezetime relay thyratron ST, and ofv the weld time relay thyratron WT, thetiming condenser 198 being normally charged by grid conduction, Uponopening of the contacts 197 the charging circuit for the condenser 198is opened and the'condenser proceeds to discharge via the resistances199 and 200, the latter resistance being variable in magnitude toestablish'variab'le hold'times for the hold time thyratron HT. After thecondenser 198 has discharged to a sufficient extent, the thyratron HT,which receives its anode potential from either the line or the line 174,via either the contacts 201a or 201b of the manual two-position switchR, in accordance with whether Repeat or Non-Repeat operation is desired,and thence via line 202 and normally closed contacts 203, fires,signifying end of hold time;

Assuming Non-Repeat operation, contacts 2011: are closed and contacts201a open, and off-time thyratron OT is deprived of voltage, taking nopart in the sequencing operation. The firing of the hold time thyratronHT energizes the hold time delay relay HTD, opening the normally closedcontacts 204, 205', 206 and 207. V

The contacts 206 are in series between the line 111 and the contacts173, thus preventing r'e-establishment of holdoif bias for the squeezetime thyratron ST by closure of the contacts 173. The contacts 205 areconnected between the line 111 and the now open contacts 197 ,Wl'llCh,when closed, serve to establish hold-off bias for the hold timethyratron HT. Accordingly, opening of the contacts 205 preventsre-establishment of hold-01f bias for the hold time thyratron HT,'evenshould the weld time relay be de-energized, closing contacts 197.Opening of the normally closed contacts 204 dis-establishes the off-biascircuit 208 for the control electrode of the off-time thyratron OT, andwhich serves normally to establish holdoff bias therefor. Accordingly,opening of the contacts 204 serves to initiate discharge of the'timingcondenser 209 over the timing resistors'210 and 211, the latter beingvariable to provide for adjustment of off times, and after apredetermined time the condenser 208 discharges over the resistors 210and 211 to an extent sufiicient to remove hold-ofi bias for the controlelectrode of the off-time thyratron OT, which may then be overcome bythe positive peaks of alternating current present at the terminal point212 between the resistors 213 and 214, which are connected across thelines 177 and 112. However, the oti-time thyratron OT has an open anodecircuit at the switch R, and cannot fire.

Energization of the hold time relay HTD additionally opens the contacts207, which are normally closed, and which normally maintain a circuitfor energizing the control relay CR. Accordingly, when hold timeterminates, as signified by energization of the hold time relay HTD,

the control relay CR is de-energized, reopening the normally opencontacts 171 and thereby de-energizing the squeeze time delay relay STD.The normally open contacts 172 are likewise now opened, in response tode energization of control relay CR, breaking the holding circuit forthe control relay CR at a further point, so that the relay CR willremain de-energized regardless of the condition of the contacts 207 atsucceeding times. Opening of contacts 172 breaks, as Well, the line 174,removing anode potential from thyratron HT. The contacts 173 are closedby de-energization of control relay CR which permits re-establishment ofhold-off bias for the squeeze time thyratron ST. De-energizati'on of thesqueeze time delay relay STD likewise closes the normally closedcontacts 193, to enable re-establishment of a hold-off bias on thecontrol electrode 162 of the weld time thyratron WT.

De-energization of the squeeze time delay relay STD, further opens thenormally open contacts 187, removing anode potential from the weld timethyratron WT, and thereby current from the weld time delay relay WTD,enabling closure of the normally closed contacts 197 and 163.

De-energization of the squeeze time delay relay STD, additionally,enables closure of contacts 113 and deenergization of weld time delayrelay WTD accomplishes closure of contacts 163, the frequency controlcircuit of Figure 3 being thereby established in operative condition,ready for a succeeding welding operation in response to further closureof the manual switch SW, and the sequence system being likewise in itsoriginal condition, ready for a further welding operation.

Should it be desired to provide repeat operation instead of thenon-repeat operation which has just been described, the double throwrepeat switch R is thrown into its alternative position, closingcontacts 201a and opening contacts 201b, providing the off-time relay OTand the off-time delay relay OTD with potential via the line 110 and theswitch R. The hold time thyratron HT and the hold time delay relay HTDare not now in circuit with line 174, but rather with line 110, viacontacts 201a, and 203. Accordingly, de-energization of control relay CRhas no effect on the hold time thyratron HT, and the latter remainsenergized until the off-time thyratron has fired, opening contacts 203,disabling hold time thyratron HT and hold time delay relay HTD. Releaseof the latter provides a grid conduction charging circuit for thyratronOT, which is thus biased 01f. Release of hold time delay relay HTDrecloses contacts 207 enabling reenergization of control relay CR andinitiation of a further weld sequence.

The two position switch R performs the function of bypassing the circuitcloser SW, and providing a circuit over line 110 for the controlthyratrons HT and OT, when in the repeat position, and supplyingoperating voltage to the hold circuit comprising the thyratron HT andthe relay HTD to the exclusion of the off relay OT and the off relay OTDwhen in the alternative position. When the switch R assumes the repeatposition, the system operates in repeated cycles, whereas in thealternative or non-repeat position the system contemplates making ofsingle welds. The latter operation requires no offtime, and each weldingoperation requires a separate closure of the switch SW. The repeatoperation on the other hand requires the inter-position of an off periodbetween the hold and the squeeze period.

It is to be understood that the present invention is not limited to theparticular details described above, since many equivalents for thespecific elements and arrangements utilized in the above disclosure willsuggest themselves to those skilled in the art. While the invention hasbeen disclosed as applied to a welding system, it is susceptible broadlyto use as a system of controlled transmission of power between a threephase power source and a single phase load. Additionally, the

system may be considered broadly as a frequency changing system fortranslating the fr quency of a source into a lower frequency, having avalue which may be selected at will. Various types of tubes may besubstituted for the electronic discharge devices disclosed, andmodification of the specific circuit arrangement disclosed may bedevised, which, however, incorporate the principles of operation setforth in the specific embodiment of the invention herein disclosed.

In view of the above facts, it is desired that the appended claims beaccorded a broad interpretation which is commensurate with the truespirit and scope of the invention within the pertinent art.

I claim as my invention:

1. In a timing system for controlling transmission times of current inalternately opposite directions between the phases of a three phase lineand a single phase load, a plurality of gaseous conduction devices forcontrolling transfer of current from said phases of said three phaseline to said single phase load in sequence, a relay for terminatingconduction times of said gaseous conduction device when said relay isenergized, means comprising a gaseous control valve for energizing saidrelay when said gaseous control valve is fired, and means operativeduring firing of any one of said gaseous conduction devices forpreventing firing of said gaseous control valve.

2. In a timing system for controlling transmission time of current inalternately opposite directions for equal times between the phases of athree phase line and a single phase load, a plurality of gaseousconduction devices for controlling transfer of current from said threephases of said three phase line to said single phase load in sequence ina first direction and thereafter in sequence from said three phases ofsaid three phase line to said load in a second direction, a relay, meansfor terminating said transmission time in response to energization ofsaid relay, means for initiating and measuring said transmssion timecomprising a discharge valve connected in series with said relay, andhaving first and second control electrodes, and a timing circuit fordetermining off time of said discharge valve comprising means forapplying cut-off bias to one of said control electrodes for apredetermined time, and means operative during firing of any of saidplurality of gaseous conduction devices for maintaining cut-ofi bias onthe other of said control electrodes.

3. A control system for supplying a single-phase load from a three-phasepower source with single-phase alternating current of a lower frequencythan the frequency of the power source, for a predetermined short timeinterval of the order of a few cycles of the lower frequency, wherebyeach half cycle of the lower frequency consists of a predeterminednumber of cycles of the supply frequency, especially for resistancewelding purposes, comprising at least one group of three arc dischargedevices, each of which is connected in current controlling relationbetween one phase of the power source and the load so as to supply saidload with current of one polarity, and a timing device adapted to besupplied from a single phase of said three-phase source and to delivercontrol impulses of one polarity, one each, during a predeterminednumber of periods of said source, a grid controlled electronic controltube being associated with each of said are discharge devices the anodecircuits of said control tubes each be ing connected to the same phaseof said source as the corresponding arc discharge device, each of saidcontrol tubes including, in its grid circuit, biasing means, tending tomaintain said control tubes non-conductive, and energy storage means,the energy storage means of the first control tube, associated with thefirst discharge device, being connected to said timing device to becharged thereby to condition said first control tube to becomeconductive when the potentials between its electrodes attain the propermagnitudes, said firstcontroltube being operatively connected to saidfirst discharge device and to the energy storage means of the secondcontrol tube, associated with the seocnd discharge device, so that saidfirst control tube, when rendering the first discharge deviceconductive, simultaneously charges the energy storage means ofsaidsecond control tube to condition said second control tube to becomeconductive when the potentials between its electrodes attain the propermagnitudes to render the latter conductive subsequently, said secondcontrol tube being similarly operatively connected to the seconddischarge device and to the energy storage means of the third controltube, associated with the third dischagre device, said third controltube being operatively connected only to said third discharge device.

4. In a timing system for controlling sequential transmission times ofcurrent between a first three phase line and a load, the combinationcomprising first, second and third gaseous conduction devices fortransmitting current from first, second and third phases, respectively,of said three, phase line to said load in a first predetermineddirection, fourth, fifth and sixth gaseous conduction devices fortransmitting current from said first, second and third phases,respectively of said three phase line to said load in a secondpredetermined direction, a second three phase line, phase shift meansconnected between said first line and said second line to shift thepotential of each phase of said second line by a predetermined anglewith respect to a corresponding phase of said first line, first, secondand third firing control valves for said first, second and third gaseousconduction devices, respectively, fourth, fifth and sixth firing controlvalves for said fourth and fifth andsixth gaseous conduction devices,respectively, means connecting said first and fourth firing controlvalves to a first phase of saidsecond line to be energized in oppositephase from said first phase of said second line, means connecting saidsecond and fifth firing control valves to a second phase of said secondline to be energized in opposite phase by said second phase of saidsecond line, means connecting said third and sixth firing control valvesto a third phase of said second line to be energized in opposite phaseby said third phase of said second line, first means for generating acontrol pulse for actuating said first firing control valve for firingsaid first gaseous conduction device at an instant in said first phaseof said first line corresponding to said predetermined angle for saidfirst phase of said second line, means responsive to actuation of saidfirst firing control valve for generating a second control pulse foractuating said second firing control valve for firing said secondgaseous conduction device at an instant in a period of said second phaseof said first line corresponding to said predetermined angle for saidsecond phase of said second line, means responsive to actuation of saidsecond firing control'valve for generating a third control puse foractuating said third firing control valve for firing said third gaseousconduction device at an instant in a period of said third phase of saidfirst line corresponding to said predetermined angle for said thirdphase of said second line, second means for generating a control pulsefor actuating said fourth firing control valve for firing said fourthgaseous conduction device at an instant in a period of said first phaseof said first line corresponding to said predetermined angle for saidfirst phase of said second line, means responsive to actuation of saidfourth firing control valve for generating a fifth control pulse foractuating said fifth firing control valve for firing said fifth gaseousconduction device at an instant in a period of said second phase of saidfirst line corresponding to said predetermined angle for said secondphase of said second line, and means responsive to actuation of saidfifth firing control valve for generating a sixth control pulse foractuating said sixth firing control valve for firing said sixth gaseousconduction device at an instant in a period of said third phase of saidfirst line corresponding to said predetermined angle for said thirdphase of said second line.

5. In a system for timing transfer of current between a three phase lineand a single phase load in alternately opposite directions for equaltime periods, comprising in combination first normally inoperative meansfor transferring a current pulse once from each of the several phases ofsaid three phase line to said single phase load in timed succession, ina predetermined order only, and in a first direction, second normallyinoperative means for transferring a current pulse once from each of theseveral phases of said three phase line to said single phase source intimed succession, in a predetermined order only, and in a seconddirection, first control means responsive to a first single controlpulse for rendering said first means operative and responsive tosucceeding first control pulses for further rendering said first meansoperative, further control means responsive to a second single controlpulse for rendering said second means operative, and responsive tosucceeding second single control pulses for further rendering saidsecond means operative, first means for generating a predeterminednumber of said first control pulses, said first means including a firstelectric discharge device having an anode, a cathode and a controlelectrode, a first circuit including said anode and cathode fortransmitting saidfirst control pulses, and a first time constant networkconnected to said control electrode when charged for blocking the flowof said control pulses through said circuit, second means for generatinga like predetermined number of said second control pulses, said secondmeans including a second electric discharge device having an anode, acathode and a control electrode, a second circuit including saidlastnamed anode and cathode for transmitting said second control pulsesand a second time constant network when charged blocking the flow ofsaid second control pulses through said second circuit, means includinga third electric discharge device to be rendered conducting for chargingsaid first network, means including a fourth electric discharge deviceto be rendered conducting for charging said second network, means forrendering said third device non-conducting during a first predeterminedtime interval to permit said first network to discharge and remaindischarged during at least a portion of said first interval and meansfor rendering said fourth device nonconducting during a second timeinterval to permit said second network to discharge and remaindischarged during at least a portion of said second interval.

6. In combination a plurality of pairs of supply conductors adapted tobe energized from a polyphase supply and when so energized adapted tosupply potentials displaced in phase in accordance with the phases ofthe supply, a first electric discharge device having an anode, a cathodeand a control electrode, a first time constant network, a firsttransformer having a secondary, a first rectifier, means connecting inseries said secondary, said rectifier and said network, means connectingsaid network between said control electrode and said cathode, a secondelectric discharge device having an anode, a cathode and a controlelectrode, a second time constant network, a second rectifier, a secondtransformer having a primary and a secondary, means including saidprimary connecting said anode and cathode of said first device to besupplied from a first of said pairs of conductors, means connecting inseries said secondary of said second transformers, said second rectifierand said second network, means connecting said second network betweenthe control electrode and cathode of said second device, a thirdelectric discharge device having an anode, a cathode and a controlelectrode, a third time constant network, a third rectifier, a thirdtransformer having a primary and a secondary, means including saidpimary of said third transformer connecting said anode and cathode ofsaid second device to be supplied from a second of said pairs ofconductors, means connecting in series the secondary of said thirdtransformer, said third rectifier and said third network, meansconnecting said third network between the control electrode and cathodeof said third device, and means connecting said anode and cathode ofsaid third device to be supplied from a third pair of said conductors.

7. In a timing system for controlling sequential transmission times ofcurrent between a three phase line and a single phase load, a pluralityof ignitrons for effecting transfer of current in a predeterminedsequence between the three phases of said three phase line and saidsingle phase load in a first direction, a further plurality of ignitronsfor effecting transfer of current in a predetermined sequence betweenthe three phases of said three phase line and said single phase load ina second direction, a relay, means for energizing said relay, meansresponsive to energization of said relay for initiating said transfer ofcurrent in a predetermined sequence and in a first direction between thephases of said three phase line and said single phase load, a furtherrelay, means for energizing said further relay, means responsive toenergization of said further relay for terminating said transfer in saidfirst direction, control means for disabling said means for energizingsaid further relay, and means for rendering said control means operativeonly after termination of transfer of current in a predeterminedsequence between all three phases of said three phase line and saidsingle phase load in said second direction.

8. In combination, first, second and third conductors of a polyphasesupply, first, second and third transformers, each transformer having aprimary and a secondary having an intermediate tap, means connectingsaid primary of said first transformer between said first conductor andsaid second conductor, means connecting said primary of said secondtransformer between said second conductor and said third conductor,means connecting the primary of said third transformer between saidthird conductor and said first conductor, first resistance means, secondresistance means, third resistance means, means connecting each saidresistance means between a common electrical point and the intermediatetap of a different one of said secondaries, fourth resistance means,fifth resistance means, sixth resistance means, said fourth, fifth andsixth resistance means being variable, and means connecting said fourth,fifth and sixth resistance means in a delta network with saidintermediate taps as apices.

9. In combination, first, second and third conductors of a polyphasesupply, first, second and third transformers, each transformer having aprimary and a secondary having an intermediate tap, means connectingsaid primary of said first transformer between said first conductor andsaid second conductor, means connecting said primary of said secondtransformer between said second conductor and said third conductor,means connecting the primary of said third transformer between saidthird conductor and said first conductor, first resistance means, secondresistance means, third resistance means, means connecting each saidresistance means between a common electrical point and the intermediatetap of a different one of said secondaries, fourth resistance means,fifth resistance means, sixth resistance means, said fourth, fifth andsixth resistance means being variable, means connecting said fourth,fifth and sixth resistance means in a delta network with saidintermediate taps as apices, first, second and third electric dischargedevices each having a pair of principal electrodes, means connecting aprincipal electrode of said first device to said fourth resistancemeans, means connecting a principal electrode of said second device tosaid fifth resistance means, means connecting a principal electrode ofsaid third device to said sixth resistance means, and means connectingthe other principal electrodes of said devices to said common point.

10. In an electrical controlling apparatus for supplying an electricalload from an alternating electrical source, a pair of normallyde-energized voltage producing devices, selectively energized means foralternately energizing said devices at the desired load frequency, meansfor energizing said selective means to initiate a pulsating operation ofsaid devices and including means responsive to the frequency of saidsource for synchronizing the instant of initiation of said pulsationswith respect to the voltage wave of said source, a pair of asymmetriccurrent conducting devices connected in reverse relation between saidsource and said load, and means responsive to said pulsations of one ofsaid voltage producing devices for rendering one of said asymmetricdevices conductive and responsive to said pulsations of the other ofsaid voltage producing devices for rendering the other of saidasymmetric devices conductive.

ll. In a network for supplying single phase alternating voltage to aload circuit from a polyphase source of voltage supply, frequencycontrol means for supplying pulses of voltage at twice the desiredfrequency of said load circuit, a first set of asymmetric currentconducting means individually connecting each of said source phases tosaid load circuit, a second set of asymmetric current conducting meansindividually connecting each of said source phases to said load circuit,said first set of asymmetric means being connected to supply current tosaid load circuit in a direction opposite to that of said second set ofasymmetric means, means responsive to alternate pulses of said frequencycontrol means for rendering said first set conductive, means responsiveto opposite alternate pulses of said frequency control means forrendering said second set conductive, said frequency control meansincluding means for establishing a predetermined time period between theactuation of said frequency control means and said sets.

12. In a network for supplying single phase alternating voltage to aload circuit from a polyphase source of voltage supply, a first set ofasymmetric current conducting devices individually connecting the phasesof said source to said load to pass current in one direction, a secondset of current conducting devices individually connecting the phases ofsaid source to said load to pass current in a direction opposite to saidone direction, a control circuit individual to each of the phases ofsaid source, each said circuit comprising a first control deviceoperable to render the respective said asymmetric devices of said firstset conductive and a second control device operable to render therespective said asymmetric devices of said second set conductive, meansconnecting said first devices for sequential operation and includingasignal circuit interconnecting each said preceding one of said firstdevices with the next succeeding one of said first devices whereby therendering of a predetermined first-to-be-actuated one of said firstdevices effective results in the remainder of said first devices beingsequentially rendered effective irrespective of the condition of saidasymmetric devices, means connecting said second devices for sequentialoperation and including a signal circuit interconnecting each saidpreceding one of said second devices with the next succeeding one ofsaid second devices whereby the rendering of a predeterminedfirst-to-be-actuated one of said second devices effective results in theremainder of said second devices being sequentially rendered effectiveirrespective of the condition of said asymmetric devices, means operableat the desired frequency of said load circuit and in predeterminedrelation to the phase of the voltage of one of the phases of said sourceto render said predetermined first-to-be-actuated one of said firstcontrol devices effective and to render said one predeterminedfirst-to-be-actuated one of said second control devices effective.

13. In a network for supplying single phase alternating voltage to aload circuit from a polyphase source of voltage supply, a first set ofasymmetric current conducting devices individually connecting the phasesof aid 25 source to said lead to pass current in one direction, a secondset of current conducting devices individually connecting the phases ofsaid source to: said load to pass current in a direction opposite tosaid one direction, a

control circuit individual to each of the phases of said source, eachsaid circuit comprising a first control device operable to render therespective said asymmetric devices of said first set conductive and asecond control device operable to render the respective said asymmetricdevices ofsaid second set conductive, means connecting said first,devices for sequential operation and including a signal circuitinterconnecting each said preceding one of saidfirst devices with the,next succeeding one of said first devices whereby the rendering of apredetermined first-to-be-actuated one of said first devices effectiveresults in the remainder of said first devices being sequentiallyrendered effective irrespective of the condition of said asymmetricdevices, means connecting said second devices for sequential operationand including a signal circuit interconnecting each said preceding oneof said second devices with the next succeeding one of said seconddevices whereby the rendering of a predetermined first-to-be-actuatedone of said second devices efiective results in the remainder of saidsecond devices being sequentially rendered effective irrespective of thecondition of said asymmetric devices, means operable at the desiredfrequency of said load circuit and in predetermined relation to thephase of the voltage of one of the phases of said source to render saidpredetermined firstto-be-actuated one of said one first control deviceseffective and to render said predetermined first-to-be-actuated one ofsaid second control devices effective, and means for determining thepoints in the wave of said source voltage supply that each of said firstand second devices are effective to actuate said asymmetric devices.

14. In a network for supplying single phase alternating voltage to aload circuit from a polyphase source of voltage supply, a first set ofasymmetric current conducting devices individually connecting the phasesof said source to said load to pass current in one direction, a secondset of asymmetric current conducting devices individually connecting thephases of said source to said load to pass current in a directionopposite to said one direction, a control circuit individual to each ofthe phases of said source, each said circuit comprising a first controldevice operable to render the respective said asymmetric devices of saidfirst set conductive and a second control device operable to render therespective said asymmetric devices of said second set conductive, meansconnecting said first devices for sequential operation whereby therendering of one of said first devices effective results in theremainder of said first devices being sequentially rendered effective,means connecting said second devices for sequential operation wherebythe rendering of one of said second devices effective results in theremainder of said second devices being sequentially rendered effective,means operable at the desired frequency of said load circuit and inpredetermined relation to the phase of the voltage of one of the phasesof said source to render said one first control devices effective and torender said one second control devices effective, a polyphase phaseshifting network connected to said voltage supply and having a polyphasevoltage output, circuit means individually connecting said controlcircuits to the output phases of said phase shifting network in the samephase order as the phases of said voltage supply are connected to theassociated ones of said asymmetric devices.

15. In a network for supplying electrical energy to a load, comprisingin combination, a pair of alternately energizable elements, aninterpulse timing network including a pair of timing devices, a firstcircuit interconnecting one of said interpulse' timing devices foractuation by one of said alternately energizable elements, a

second circuit interconnecting the other of said interpulsev natelyenergizable elements, a load current controlling network interconnectingsaid load to a source of electrical energy, said load current networkincluding a first and a second asymmetric current conducting means, saidfirst asymmetric meansbeing connected to conduct current to such load ina first direction, said second asymmetric means being connected toconduct current to such load in a second direction, each of saidasymmetric means including a controldevice for controlling its currentconducting condition, a third circuit connecting said control device ofsaid first asymmetric means to said one timing device of said interpulsetiming network whereby the conductive condition of said first asymmetricmeans is controlled, a fourth circuit connecting said control device ofsaid second asymmetric means to said other timing device of saidinterpulse timing network whereby the conductive condition of saidsecond asymmetric means is controlled, each said timing device of saidinterpulse timing network having two timing intervals.

16. In a network for supplying electrical energy to a load from apolyphase supply of alternating potential, comprising in combination, apair of alternately energizable elements, an interpulse timing networkincluding a pair of timing devices, a first circuit interconnecting oneof said interpulse timing devices for actuation by one of' saidalternately energizable elements, a second circuit interconnecting theother of said interpulse timing de vices for actuation by the other ofsaid alternately energizable elements, a load current controllingnetwork interconnecting such load to a source of electric energy, saidload-current network including a pair of asymmetric current controllingunits connected between each of the phases of such alternating potentialsupply'and said load, a first asymmetric unit of each of said pair or"units being polarized to conduct current to such load in a firstdirection, a second asymmetric unit of each of said pair of units beingpolarized to conduct current to such load in a second direction, saidload current controlling network further including a first and a secondbiasing means for controlling the conductive condition of each of saidasymmetric units, a third circuit connecting said first biasing means tosaid one timing device of said interpulse timing network whereby theconductive condition of said first asymmetric unit is controlled intimed relationship to the energization of one of said alternatelyenergized elements, a fourth circuit connecting said second biasingmeans to said other timing device whereby the conductive condition ofsaid second asymmetric unit is controlled in timed relationship to theenergizati-on of the other of said alternately energized elements.

17. In an apparatus controlling the flow of electrical energy, asequencing network including a first and a second energizable device, anelectrical energy flow controlling network including at least one pairof normally non conductive asymmetric current controlling devicesconnected in back-to-back relation between a source of electrical energyand a load, said second energizable device including an electric valvehaving a pair of main electrodes and a pair of controlling electrodes, atiming means having an output potential circuit connected between one ofsaid controlling electrodes and one of said main electrodes and normallymaintaining a bias potential therebetween for holding said valve nonconductive, means operatively connecting said timing means to said firstenergizable device whereby said timing means is actuatable in responseto a change in the energized condition of said first energizable deviceto remove said blocking bias potential at a desired time intervalsubsequent to actuation of said first energizable device, an outputfrequency timing device having an output network connected to controlthe conductive periods of said asymmetric devices in a conductingcondition, and a bias potential circuit connected between the other saidcontrolling electrodes and said one main electrode for applying ablocking bias potential to said valve during periods of conduction ofone of said asymmetric controlling devices.

18. In an apparatus controlling the flow of electrical energy, asequencing network including a first and a second energizable device, anelectrical energy flow controlling network controlling flow of energy ineach of two directions between a source of electrical energy and a load,said second device including an electric valve having a pair of mainelectrodes and a pair of controlling electrodes, a timing means havingan output potential circuit connected between one of said controllingelectrodes and one of said main electrodes and normally maintaining abias potential therebetween for holding said valve non conductive, meansoperatively connecting said timing means to said first energizabledevice whereby said timing means is actuatable in response to a changein the energized condition of said first energizable device to removesaid blocking bias potential at a desired time interval subsequent toactuation of said first energizable device, an output frequency timingdevice having an output network connected to control said energy flowcontrolling network and operable to alternately actuate the same toprovide for an alternating fiow of current between said source and saidload, and a bias potential circuit connected between the other saidcontrolling electrodes and said one main electrode for applying ablocking bias potential to said valve during flows of current betweensaid load and said source in one direction.

19. In an apparatus controlling the flow of electrical energy, asequencing network including a first and a second energizable device, anelectrical energy flow controlling network controlling fiow of energy ineach of two directions between a source of electrical energy and a load,said second device including an electric valve having a pair of mainelectrodes and a conductivity controlling means, a timing means havingan output potential circuit connected between said valve conductivitycontrolling means and one of said main electrodes and normallymaintaining a bias potential therebetween for holding said valve nonconductive, means operatively connecting said timing means to said firstenergizable device whereby said timing means is actuatable in responseto a change in the energized condition of said first energizable deviceto remove said blocking bias potential at a desired time intervalsubsequent to actuation of said first energizable device, an outputfrequency timing device having an output network connected to controlsaid energy flow controlling network and operable to alternately actuatethe same to provide for an alternating flow of current between saidsource and said load, and a bias potential circuit connected betweensaid valve conductivity controlling means and said one main electrodefor applying a blocking bias potential to said valve during flows ofcurrent between said load and said source in one direction.

20. In a timing system for controlling sequential transmission times ofcurrent between a first three phase line and a single phase load,comprising in combination a second three phase line, phase shift meansconnecting said first three phase line to said second three phase lineto shift the potential of each phase of said second line by apredetermined angle with respect to the corresponding phase of saidfirst line, means connected to a first phase of said second line to beactuated and when so actuated for initiating transmission of currentbetween a corresponding first phase of said first line and said load,said initiation taking place at an instant in a period of said firstphase corresponding to said predetermined angle for said first phase,means connected to a second phase of said second line actuableresponsive to said initiation of transmission for sequentiallyinitiating transmission of current between a corresponding second phaseof said first line and said load, said initiation taking place at aninstantin a period of said second phase corresponding to saidpredetermined angle for said second phase, means connected to a thirdphase of said second line actuable responsive to initiation oftransmission of current between said second phase of said first line andsaid load for sequentially initiating transmission of current between acorresponding third phase of said first line and said load, saidinitiation taking place at an instant in a period of said third phasecorresponding to said predetermined angle for said third phase, andtiming means connected to said means connected to said first phase ofsaid second line for actuating said last named means connected asaforesaid during predetermined time intervals, said timing meansinitiating the transmission of current only between said first phase ofsaid first line and said load at the beginning of each of saidintervals.

21. In a timing apparatus, a first timing circuit including a firstelectric valve normally held in a nonconducting condition and a firstdischargeable energy storage network, a second timing circuit includinga second electric valve normally held in a nonconducting condition and abiasing network for controlling its conductive condition, said biasingnetwork including a second dischargeable energy storage network, meansfor rendering said first electric valve conducting, means responsive tothe conduction of said first valve for initiating the timing out of bothsaid storage networks, means interconnecting said second storage networkwith said second electric valve whereby upon the timing out of saidsecond network said second electric valve will become conductive, andmeans responsive to the timing out of said first storage network foractuating said biasing network to render said second valvenonconductive.

22. In a timing apparatus, a first electric valve having a pair of mainelectrodes and control means, a bias network connected between saidcontrol means and one of said main electrodes, said network comprising adischargeable energy storage circuit connected between said controlmeans and said one main electrode such that when said circuit isenergized said valve is held nonconductive, means normally maintainingsaid circuit energized, a second electric valve having a pair of mainelectrodes and a control electrode, a second dischargeable energystorage circuit connected between said control electrode and one of saidmain electrodes of said second valve, means normally maintaining saidsecond circuit energized, means for substantially concurrentlyterminating the flow of energy to said circuits, and means including atleast a portion of said bias network and responsive to the discharge ofsaid second circuit to a predetermined value to render said first valvenonconducting.

23. A control system for supplying a single-phase load from athree-phase power source with single-phase alternating current of alower frequency than the frequency of the power source, for apredetermined short time in terval of the order of a few cycles of thelower frequency, whereby each half cycle of the lower frequency consistsof a predetermined number of cycles of the supply frequency, especiallyfor resistance welding purposes, comprising at least one group of threeare discharge de vices, herein designated as first, second and thirddevices, each of which is connected in current controlling rela tionshipbetween one phase of the power source and the load so as to supply saidload with current of one polarity and a second group of three dischargedevices, herein designated as fourth, fifth and sixth devices, eachdevice. of the second group being connected in anti-parallel with acorresponding device of the first group, and a timing device adapted tobe supplied from a single-phase of said three-phase source and todeliver control impulses of one polarity, one each for each group ofdischarge devices, during a pretei-mined number of periods of saidsource, a grid controlled electronic control tube being associated witheach of said are discharge devices, the anode circuits of said controltubes each being connected to the same phase of said source as thecorresponding arc discharge device, each of said control tubesincluding, in its grid circuit, biasing means, tending to maintain saidcontrol tube non-conductive, and energy storage means, the energystorage means of the first control tube and the fourth control tube,associated with the first discharge device and the fourth dischargedevice respectively, being connected to said timing device to be chargedthereby to condition said first control tube and fourth control tube tobecome conductive when the potentials between its electrodes attain theproper magnitudes, said first control tube and fourth control tube beingoperatively connected to said first discharge device and fourthdischarge device and to the energy storage means of the second controltube and the fifth control tube, associated with the second dischargedevice and the fifth discharge device respectively, so that said firstcontrol tube and said fourth control tube, when rendering the firstdischarge device and the fourth discharge device respectivelyconductive, simultaneously charge the energy storage means of saidsecond control tube and said fifth control tube to condition said secondcontrol tube and said fifth control tube to become conductive when thepotentials between their respective electrodes attain the propermagnitudes to render the latter conductive subsequently, said secondcontrol tube and said fifth control tube being similarly operativelyconnected to the second discharge device and the fifth discharge devicerespectively and to the energy storage means of the third control tubeand the sixth control tubes respectively, associated with the thirddischarge device and the sixth discharge device respectively, said thirdcontrol tube and said sixth control tube being operatively connectedonly to said third discharge device and said sixth discharge devicerespectively.

24. A control system for supplying a single-phase load from athree-phase power source with single-phase alternating current of alower frequency than the frequency of the power source, for apredetermined short time interval of the order of a few cycles of thelower frequency, whereby each half cycle of the lower frequency consistsof a predetermined number of cycles of the supply frequency, especiallyfor resistance welding purposes, comprising at least one group of threeare discharge devices, herein designated as first, second and thirddevices, each of which is connected in current controlling relationshipbetween one phase of the power source and the load so as to supply saidload with current of one polarity and a second group of three dischargedevices, herein designated as fourth, fifth and sixth devices, eachdevice of the second group being connected in anti-parallel with acorresponding device of the first group, and a timing device adapted tobe supplied from a single-phase of said three-phase source and todeliver control impulses of one polarity, one each for each group ofdischarge devices, during a predetermined number of periods of saidsource, a grid controlled electronic control tube being associated witheach of said arc discharge devices, the anode circuits of said controltubes each being connected to the same phase of said source as thecorresponding arc discharge device, each of said control tubesincluding, in its grid circuit, biasing means, tending to maintain saidcontrol tube non-conductive, and energy storage means, the energystorage means of the first control tube and the fourth control tube,associated with the first discharge device and the fourth dischargedevice respectively, being connected to said timing device to be chargedthereby to condition said first control tube and fourth control tube tobecome conductive when the potentials between its electrodes attain theproper magnitudes, said first control tube and fourth control tube beingoperatively connected to said first discharge device and fourthdischarge device respectively and to the energy storage means of thesecond control 3o tube and the fifth control tube respectivelyassociated with the second discharge device and the fifth dischargedevice respectively, so that saidfirst control tube and said fourthcontrol tube, when rendering the first discharge device and the fourthdischarge device respectively conductive, simultaneously charges theenergy storage means of said second control tube and said fifth controltube respectively to condition said second control tube and said fifthcontrol tube to become conductive when the potentials between itselectrodes attain the proper magnitudes to render the latter conductivesubsequently, said second control tube and said fifth control tube beingsimilarly operatively connected to the second discharge device and thefifth discharge device respectively and to the energy storage means ofthe third control tube and the sixth control tube respectively,associated with the third discharge device and the sixth dischargedevice respectively, said third control tube and said sixth control tubebeing operatively connected only to said third discharge device and saidsixth discharge device respectively, said timing device being adjustableto supply a predetermined number of control impulses in succession tosaid one group of discharge devices, to supply subsequently the samenumber of control impulses in succession to said second group ofdischarge devices, and to repeat this whole cycle of control impulses apredetermined number of times.

25. A control system for supplying a single-phase load from athree-phase power source with single-phase alternating current of alower frequency than the frequency of the power source, for apredetermined short time interval of the order of a few cycles of thelower frequency, whereby each half cycle of the lower frequency consistsof a predetermined number of cycles of the supply frequency, especiallyfor resistance welding purposes, comprising at least one group of threearc discharge devices, herein designated as first, second and thirddevices, each of which is connected in current controlling relationshipbetween one phase of the power source and the load so as to supply saidload with current of one polarity and a second group of three dischargedevices, herein designated as fourth, fifth and sixth devices, eachdevice of the second group being connected in anti-parallel with acorresponding device of the first group, and a timing device adapted tobe supplied from a single-phase of said three-phase source and todeliver control impulses of one polarity, one each for each group ofdischarge devices, during a predetermined number of periods of saidsource, a grid controlled electronic control tube eing associated witheach of said are discharge devices, the anode circuits of said controltubes each being connected to the same phase of said source as thecorresponding arc discharge device, each of said control tubesincluding, in its grid circuit, biasing means, tending to maintain saidcontrol tube non-conductive, and energy storage means, the energystorage means of the first control tube and the fourth control tube,associated with the first discharge device and the fourth dischargedevice respectively, being connected to said timing device to be chargedthereby to condition said first control tube and fourth control tube tobecome conductive when the potentials between its electrodes attain theproper magnitudes, said first control tube and fourth control tube beingoperatively connected to said first discharge device aud fourthdischarge device respectively and to the energy storage means of thesecond control tube and the fifth control tube respectively, associatedwith the second discharge device and the fifth discharge devicerespectively, so that said first control tube and said fourth controltube, when rendering the first discharge device and the fourth dischargedevice conductive, simultaneously charges the energy storage means ofsaid second control tube and said fifth control tube respectively tocondition said second control tube and said fifth control tuberespectively to become conductive when the potentials between itselectrodes attain the proper magnitudes to render the latter conductivesubsequently, said second control tube and said fifth control tube beingsimilarly operatively connected to the second dischange device and thefifth discharge device respectively and to the energy storage means ofthe third control tube and the sixth control tube respectively,associated with the third discharge device and the sixth dischargedevice respectively, said third control tube and said sixth control tubebeing operatively connected only to said third discharge device and saidsixth discharge device respectively, said timing device being adjustableto supply a predetermined number of control impulses in succession toone group of discharge devices, to supply subsequently the same numberof control impulses in succession to the second group of dischargedevices, and to repeat this whole cycle of control impulses apredetermined number of times, said system also including a weld-timerelay adapted to terminate the flow of current to said load by renderingsaid timing device inoperative, a control circuit adapted to conditionsaid relay to operate, and interlock means between said timing deviceand said control circuit adapted to permit said relay to cause thetiming device to become inoperative only after said timing device hascompleted the delivery of all control impulses of the full cycle of suchimpulses during any part of which said relay is conditioned to operate.

26. A control system for supplying a single-phase load from athree-phase power source with single-phase alternating current of alower frequency than the frequency of the power source, for apredetermined short time interval of the order of a few cycles of thelower frequency, whereby each half cycle of the lower frequency consistsof a predetermined number of cycles of the supply frequency, especiallyfor resistance welding purposes, comprising at least one group of threearc discharge devices, herein designated as first, second and thirddevices, each of which is connected in current controlling relationshipbetween one phase of the power source and the load so as to supply saidload with current of one polarity and a second group of three dischargedevices, herein designated as fourth, fifth and sixth devices, eachdevice of the second group being connected in antiparallel with acorresponding device of the first group, and a timing device adapted tobe supplied from a singlephase of said three-phase source and to delivercontrol impulses of one polarity, one each for each group of dischargedevices, during a predetermined number of periods of said source, a gridcontrolled electronic control tube being associated with each of saidarc discharge devices, the anode circuits of said control tubes eachbeing connected to the same phase of said source as the correspondingare discharge device, eachof said control tubes including, in its gridcircuit, biasing means, tending to maintain said control tubenon-conductive, and energy storage means, the energy storage means ofthe first control tube and the fourth control tube, associated with thefirst discharge device and the fourth discharge device respectively,being connected to said timing device to be charged thereby to conditionsaid first control tube and fourth control tube to become conductivewhen the potentials between its electrodes attain the proper magnitudes,said first control tube and fourth control tube being operativelyconnected to said first discharge device and fourth discharge devicerespectively and to the energy storage means of the second control tubeand fifth control tube respectively, associated with the seconddischarge device and the fifth discharge device respectively, so thatsaid first control tube and said fourth control tube, when rendering thefirst discharge device and the fourth discharge device respectivelyconductive, simultaneously charges the energy storage means of saidsecond control tube and said fifth control tube respectively tocondition said second control tube and said fifth control tube to becomeconductive when the potentials between its electrodes attain the propermagnitudes to render the latter conductive subsequently, said secondcontrol tube and said fifth control tube being similarly operativelyconnected to the second discharge device and the fifth discharge devicerespectively and to the energy storage means of the third control tubeand the sixth control tube respectively, associated with the thirddischarge device and the sixth discharge device respectively, said thirdcon trol tube and said sixth control tube being operatively connectedonly to said third discharge device and said sixth discharge devicerespectively, said timing device being adjustable to supply apredetermined number of control impulses in succession to one group ofdischarge devices, to supply subsequently the same number of controlimpulses in succession to the second group of discharge devices, and torepeat this whole cycle of control impulses a predetermined number oftimes, said system also including a weld-time relay adapted to termiatethe flow of current to said load by rendering said tuning deviceinoperative, a control circuit adapted to condltion said relay tooperate, said control circuit including an additional control tube forcontrolling said relay, and interlock means between said timing deviceand said control circuit adapted to permit said relay to cause thetlming device to become inoperative only after said timing device hascompleted the delivery of all control impulses of the full cycle of suchimpulses during any part of which said relay is conditioned to operate,said timing device including an electronic valve and a ClfCUltcontrolled by said valve and controlling said additional control tube,said interlock means comprising an off-biasing connection for saidadditional tube provided by said circuit during the time when said valveis conducting so as to prevent said relay from being actuated duringsaid last-named time.

27. A control system for supplying a single-phase load from athree-phase power source with single-phase alternatlng current of alower frequency than the frequency of the power source, for apredetermined short time interval of the order of a few cycles of thelower frequency, whereby each half cycle of the lower frequency consistsof a predetermined number of cycles of the supply frequency, especiallyfor resistance welding purposes, comprising at least one group of threeare discharge devices, herein designated as first, second and thirddevices, each of which is connected in current controlling relationshipbetween one phase of the power source and the load so as to supply saidload with current of one polarity and a second group of three dischargedevices, herein designated as fourth, fifth and sixth devices, eachdevice of the second group being connected 1n anti-parallel with acorresponding device of the first group, and a timing device adapted tobe supplied from a single-phase of said three-phase source and todeliver control impulses of one polarity, one each for each group ofdischarge devices, during a predetermined number of periods of saidsource, a grid controlled electronic control tube being associated witheach of said are discharge devices, the anode circuits of said controltube each being connected to the same phase of said source a thecorresponding arc discharge device, each of said control tubesincluding, in its grid circuit, biasing means, tending to maintain saidcontrol tube non-conductive, and energy storage means, the energystorage means of the first control tube and the fourth control tube,associated with the first discharge device and the fourth dischargedevice respectively, being connected to said timing device to be chargedthereby to condition said first control tube and fourth control tuberespectively to become conductive when the potentials between itselectrodes attain the proper magnitudes, said first control tube andfourth control tube being operatively connected to said first dischargedevice and fourth discharge device respectively and to the energystorage means of the second control tube and the fifth control tuberespectively, associated

